The invention is related to the art of rendering images. The invention is applicable where, for example, descreened halftone or gray halftone images are to be rehalftoned. The invention will be described in relation to a xerographic environment. However, those of skill in the art will understand that the invention can be applied in other digital imaging applications. For example, the invention can be applied in lithographic, ionographic and ink jet environments.
Many gray halftone images must be transformed to binary form in order to provide signals that can drive an imager. For example, when a halftone image is scanned by a photocopier, the scanning process creates a gray halftoned version of the image. Most rendering devices are binary in nature. Therefore, the gray halftoned version of the image must be rehalftoned before a copy of the image can be made by a rendering device, such as, for example, a xerographic print engine. Other sources of gray halftone images include, but are not limited to, JPEG uncompressed halftone images and multi-level halftoned images.
A problem associated with rendering such rehalftoned images is the creation of moiré due to the beating of frequency components in a rehalftoning screen with frequency components in the original halftone screens. Therefore, one approach to rendering images that must be rehalftoned is to first descreen the image and then rehalftone. The descreening process tends to filter or average out the original halftone screen. However, descreening is not always desirable. For instance, descreening can introduce an undesirable blur into an image. Furthermore, descreening does not always completely eliminate the original halftone pattern. Therefore, a residual halftone screen component can remain in the image and rehalftoning can yield objectionable moiré.
Another approach is to simply rehalftone the gray halftoned image. As explained by David Blatner, Glen Fleischman, and Steve Roth in their book Real World Scanning and Halftones, Peach Pit Press, 1998, page 280, in simply rehalftoning the gray halftoned image, “the idea is to have an integral relationship between the original's screen frequency and the output frequency (1:1, 2:1, etc.).” However, if the rehalftone screen frequency is not an exact integral of the original's screen frequency, or if the screen angles don't exactly match, objectionable, low frequency moiré are likely to be produced.
Other approaches, such as, stochastic screening and error diffusion, often yield fragmented halftone dots that appear noisy and have high dot gain. Fragmented dots also do not render colors and gray tones as consistently as when the dots are clustered. Clustered dots tend to yield prints that are more consistent in color across a page and from print to print.
Rehalftoning at integrals of the original halftone screen frequency is suggested so all strong beats between the rehalftone screen and the original screen will occur at zero frequency. This strategy works well for applications where all screen frequencies and screen angles are known exactly and are achievable. However, this strategy is very poor if there is some uncertainty or inability to exactly achieve the required screen frequencies and angles. A small deviation in either screen frequency or angle from an exact integer multiple of an original screen frequency is likely to yield objectionable, low frequency moiré.
Unfortunately, in many rehalftoning applications, the exact frequency and angle of an original halftone screen is unavailable. For example, in a photocopying environment where a user simply places an image on a scanner and requests that copies be made, information is usually unavailable as to how the image was originally halftoned. Additionally, the angle at which the image is placed on the scanner may also be unknown. In addition, paper shrinkage occurs in many printing processes and often the shrinkage varies across a sheet of paper. Knowledge of the exact original halftone frequency does not fully describe the complicated frequency content that can occur on a scanned page. Therefore, selecting a rehalftone screen that meets the criteria of integral multiples of original screen frequency can be problematic.
Therefore, there is desire to provide a method for rendering descreened and gray halftone images that does not introduce objectionable moiré and does not produce significant dot fragmentation.